1. Field of the Invention
The present invention relates to: a sheet carrying unit which stops carrying of a sheet based on detection result of double feed of sheets; an image forming apparatus which comprises the sheet carrying unit; and a sheet carrying control method of stopping carrying of a sheet based on detection result of double feed of sheets.
2. Description of Related Art
In an image forming apparatus for forming an image on recording paper, such as a printer or a complex machine having a plurality of functions such as a scanning function, a printing function and a facsimile function, the recording paper may not be carried separately one by one due to the humidity, the static electricity or the like and double feed, which means carrying of two or more sheets of recording paper overlapped with each other, may possibly take place during carrying of recording paper from a paper cassette or a manual paper feeding tray.
Therefore, a double feed sensor for detecting double feed is provided to stop carrying of recording paper once and stop an image forming process including an exposure process, a development process, a transfer process, a fixing process and the like, and then eject the recording paper, when double feed is detected. Used as a double feed sensor is, for example, an ultrasonic sensor. Since the output level of an ultrasonic sensor changes depending on air space formed between one sheet of recording paper and another, it is possible to detect double feed of recording paper based on a change in the output level (see Japanese Patent Application Laid-Open No. 2004-4181). When an ultrasonic sensor is used wherein the recording paper is kept in a noncontact state, it is possible to detect double feed without disturbing carrying of recording paper and regardless of the thickness of the recording paper.
However, when recording paper having air space inside thereof, such as an envelope, is to be carried, there is a problem that double feed is detected even in carrying of only one sheet and the printing process is stopped, since double feed of recording paper is detected based on a change in the output level of the ultrasonic sensor depending on air space between a sheet of recording paper and another. Moreover, in addition to an envelope, double feed is also detected in carrying of only one sheet and the printing process is stopped when recording paper has air space inside thereof as in an OHP (OverHead Projector) sheet made by appending and attaching a protective sheet, Kent paper made by pasting two sheets together, or the like. Moreover, double feed is also detected in carrying of only one sheet and the printing process is stopped for recording paper such as WASHI (Japanese paper) having air space distributed therein. Therefore, a problem that an image forming process cannot be executed occurs.